Radio-frequency testing circuit



June 13, 1950 C. E. DENNIS Original Filed March 1, 1946 IND/0A TOR T 506 5/ .7 5Z\ 8 Osc. Source Osc. Source Osc.-Source Power 8 Phase L 8Phase L 8 Phase supp'y Angle 0 Ang|e l20 I Angle 240 CHAR/.55 E DENNISPatented June 13, 1950 than UNITED STATES PATENT OFFICE 1949, Serial No.97,871

11 Claims.

Divided and this application June 3,

(Granted under the act of March 3, 1883, as amended April 30, 1928; 3700. G. 757) This invention relates to a radio-frequency testing circuit,and more particularly to' that .type of testing circuit that indicatesthe deviation of frequency-sensitive electrical elements from apredetermined standard.

This application is a division of application Serial No. 651,415, filedMarch 1, 1946, now Patent No. 2,472,096, for Radio-frequency testing"circuit.

An object of this invention is to provide 'a radio-frequency testingcircuit which will rapidly produce accurate indication of the magnitudethat the interelectrode capacitances of vacuum tubes differ from thoseof a prescribed standard tube.

Another object of this invention is to provide a radio-frequency testingcircuit that utilizes three oscillating voltages, each 120 outof phasewith the other but of equal voltage amplitude and of the same selectedfrequency.

Another object of this invention is to provide a radio-frequency testingcircuit that will require personnel engaged in operating the device tomake only an initial adjustment in order to make a desired series oftests.

A further object of this invention'is .to provide a testing device ofthe variety described 'of simple design in' which the circuit elementsare limited to condensers, resistors, and electronic tubes, and in whichno inductances are required.

Other objects and improvements over prior art will be apparent from thefollowing description.

Reference is now made to the accompanying rawings in which:

Fig. 1 is a circuit diagram of the radiofrequency testing device;

Fig. 2 is a vector diagram representing the input voltages required;

Fig. 3 is a vector diagram showing voltage amplitude and phaserelationships of various voltages present in the invention during'adescribed condition of operation; and

Fig. 4 is another vector diagram of voltages present in theinvention'during another described condition of operation.

The invention comprises three vacuumatubes connected as cathodefollowers, each of which are fed a single phase of the three requiredoscillating voltages of 120 phase difierence. The source of theoscillating voltages may be a common source providing the three requiredoutputs, or may be individual sources providing the desired oscillatingvoltages. Each ortwo of .:these cathode follower tubes cathode couplesthe'volt age-impressed on its grid to two tubes connected 2 asamplifiersrespectively, While the third cathode follower tube, which isthe tube to be tested, cathode couples the voltage impressed on its gridto both of the amplifier tubes. It is here pointed out that theoscillating voltages fed to the first two cathode follower tubes mayhave any phase relationship provided their amplitudes are equal andprovided the phase'angle of the third oscillating voltage bisects thephase angle formed by the first two oscillating voltages. It is also notnecessary that the third oscillating voltage be equal in amplitude tothe first two oscillating voltages. The forthcoming analysis will applyonly to'the specific case wherein the three oscillating voltages are ofequal amplitude and phase; however, an analysis applicable to any threeoscillating voltages meeting the requirements stated above will parallelthis forthcoming analysis. The deviation of the tube under test from apredetermined standard produces a phase shift in the common voltage thatis cathode coupled to both amplifier tubes. This phase shift will causeunequal currents to flow in the amplifier tubes causing a deflection ofa direct-current indicator connected for differential action between theplates of the two amplifier tubes. Proper adjustment of a shuntingpotentiometer across this direct-current indicator to provide the propersensitivity will produce deflections to indicate percentage deviationirom standard as read from a previously calibrated scale of theindicator.

In the embodiment of Fig. 1, it is shown that the radio-frequencytesting device consists generally of the vacuum tubes 9, it, 23, (iiiand 3? in addition to the associated circuits. The five vacuum tubesshownherein in the form of triodes may have any number of elements; andit is to beunderstood that this invention is not to be limited to tubesof the triode classification. The grid ll of the tube 9 is connected tothe zerophase voltage 6 of the oscillating source 53. The plate 12 isconnected directly tothe positive terminal of a high voltage,direct-current plate voltage supply #9. The load resistance tor the tube9 is the series combination 0f ,iesistO1S i3 and 2| connected betweenthe cathode it and ground, thejunction between the two resistors beingconnected to the cathode ll of the tube it. The plate 20 of tube 16 isconnected to the power supply 49 through the load resistor 55. Acondenser It .connecting the two plates [2 and 253 provides aradio-frequency path to ground for the plate 293. Ina like manner, thegrid 32 of the tube 30 is connected ,to the 240 phase voltage 8 of theoscillatingsource 52. The .plate 33 is connected directly to thepositive terminal of the power supply 49, The load resistance for thetube 3|! comprises the series combination of resistors 21 and 34connected between the cathode 3| and ground, the junction between thetwo resistors being connected to the cathode 24 of the tube 23. Theplate 25 of tube 23 is connected to the positive terminal of the powersupply 43 through the resistor 28, the plate 26 being also connected tothe plate 33 by means of a condenser 29 acting as a radio-frequencyby-pass to ground. The grid 39 of tube 3'! is connected to the 120 phasevoltage I of the oscillating source through test terminal 46, the plate40 being directly connected to the positive terminal of the power supply49 through test terminal 45. A test terminal 44 is provided at thecathode 38, there being a non-reactive impedance 41 inserted betweenterminal 44 and ground. It is thus seen that terminal 44 and impedance4": provide a return circuit for the cathode 33 of tube 31 under testthrough power supply 49. The tube 31 to be tested is connected to testterminals 44, 45, 43, as shown in Fig. 1, there being a directconnection of grids I9 and 25 with the cathode 38. The plate 20 isconnected to an arm 4| of the potentiometer 42 which is connected acrossthe terminals of a direct-current indicator 43, whereas the plate 26 isconnected directly to one terminal of the indicator 43, thereby connecteing said indicator 43 for difierential action, that is responsive to thedifference in plate currents between tubes l6 and 23.

In operation, the cathode circuits of the tubes 9 and 33 provide cathodecoupling of the voltages 6 and 8 to the tubes I6 and 23 respectively.The phase relationship of the voltage on the cathode 38 of the tube 31to the voltage on the grid 39 will depend on the interelectrodecapacitance of tube 31 under test which will result in a and no currentwill flow between them, causing the indicator 43 to remain at itsmid-zero position.

If the voltage across impedance 41 were not phase displaced from voltage1 supplied to grid 39 of tube 31, it is apparent that this voltage oncathode 38 of tube 31 would be displaced 120 from each of voltages 6 and8 on cathodes l1 and 24,

' respectively. This voltage on the cathode 38 is phase shift of thefrequency of the oscillating voltage I on grid 39. The voltage on thecathode 38 is cathode coupled to the grids I9 and 25. The tubes 5 and 23then vectorially subtract the oscillating voltages 6 and 8 from thevoltage on the cathode 33. The resulting grid-to-cathode voltages on thetubes l6 and 23 control the plate current of these tubes, therebyvarying the directcurrent voltage at the plates 29 and 25. Thisdifference in voltage on the plates 25 and 25 causes the direct-currentindicator 43 to deflect, indicating the characteristics of the tubeunder test.

More specifically, the oscillating voltage 6 shown as a vector 6 in Fig.2 maintains this phase relationship as it appears on the cathode l1.

This voltage appearing on the cathode I1 is represented by a vector 6'in Fig. 3 and Fig. 4. Similarly, the oscillating voltage 8 shown as avector 8 in Fig. 2 maintains its phase relationship as it appears on thecathode 24 and is represented by a vector 8 in Fig. 3 and Fig. 4. Thevoltages thus impressed on the cathodes 11 and 24 cause a direct currentto fiow through the tubes I6 and 23 and through their respective loadresistors l5 and 28, the oscillating components of this direct currentbeing filtered out by the radiofrequency by-pass condensers l4 and 29 toground. The magnitude of these direct currents will be dependent uponthe characteristics of the particular vacuum tubes employed. In theabsence of other signals applied to the tubes I5 and 23, the platecurrents in both tubes will be equal, thereby causing equal voltagedrops across their respective load resistors I5 and 28. The voltages 0nthe plates and 26 will therefore be equal will be equal.

represented by a vector 5! in Fig. 4. The effective grid l9 to cathodevoltage of the tube It will be the vector difference between the voltageon the grid l5 and the voltage on the cathode IT; as according to Fig.3, the vector difierence between vector 5| and vector 6 respectively.This vector difierence is equivalent to the vector summation of thevector 5| and vector 52 which is equal in amplitude but opposite indirection to the vector 6. The resulting grid I!) to cathode voltage isnow represented by a vector 54.

Similarly, the efiective grid to cathode 24 voltage of the tube 23 willbe the vector difference between the voltage on the grid 25 and thevoltage on the cathode 24; as according to Fig. 3, the vectordifi'erence between vector 5| and vector 8' respectively. This vectordifference is equivalent to the vector summation of the vector 5| and avector 53 which is equal in amplitude but opposite in direction to thevector 8. The resulting grid 25 to cathode 24 voltage is now representedby the vector 55. Thus it can be seen that when there is no phasedisplacement in the circuit of tube 31, the vector 5| remains 120 fromboth vectors 6 and 8', and the vectors 54 and 55 This condition willcause equal direct currents to flow in the tubes l6 and 23, equal dropsacross their respective load resistors |5 and 28, and hence, equalvoltages to be present on the plates 20 and 26. The equal voltages onthe plates 20 and 25 will cause no deflection of the indicator 43connected between the two plates 29 and 25, and the indicator needlewill remain at its mid-zero position indicating standardization.

When the tubes under test are substituted for the standardizing tube,any deviations of the interelectrode capacitance will result in a phasedisplacement of vector 5| from vector 1. This displacement may be eitherlagging or leading, and the lagging condition will be considered first.This shift is represented in Fig. 4 by the vector 5| rotated from vector5| of Fig. 3 in a clockwise direction, indicating a voltage lag. Theeiiective grid-to-cathode voltages of the tubes l6 and 23 will now bethe vector summations of the vector 52 with the shifted vector 5| and ofthe vector 53 with the shifted vector 5| respectively. The resultingvectors 54 and 55 thus produced can be seen to be unequal in amplitudethus causing unequal currents to flow in the tubes l6 and 23 and theirrespective load resistors l5 and 28. Resulting from this condition,unequal voltages will appear on the plates 20 and 26, whereby theindicator 43 will be caused to deflect in one direction from itsmid-zero position. When the tube under test results in a voltage leadingin phase with respect to voltage I, the vector 5| will lead thenon-reactive vector 5| of Fig. 3, a similar inequality of plate voltageson the tubes l6 and 23 exists, and the indicator 43 will again be causedto deflect but in the opposite direction from its mid-zero position.

For calibration of the indicator, the standardizing tube is again placedin the circuit. The frequency of the oscillating sources 50, 5| and 5;

gamed shifted a i k nown percentage, and the potentiometer 42 isadjusted thereby altering the sensitivity of theindicator 43 until theindicator l3 reads the same percentage on the, calibrated scale, Forgreater accuracy in calibration, the frequency of theoscillating'sources 50, 5| and 52 "may be shifted the same percentage inthe opposite direction, actuating the indicator in the oppositedirection. If necessary, readjustment can then be made with thepotentiometer 42 to obtain a deflection corresponding to the knownpercentage frequency deviation. The oscillating source is then,readjusted to the or'igi'nalfrequency whereupon the meter shouldread atits mid-Zero position. A tube to be tested can now bes'ubstituted forthe's'tandardizing tube, and the amount and direction of the deviationcan now be read directly'from the calibrated scale, therebypresenting'an indication of the relative magnitudes of theinterelectrode capacitances of the standard tube and the tubes undertest.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed is:

1. In a radio-frequency testing circuit, comprising: a first amplifyingtube having a cathode a control grid and a plate; a first vacuum tubehaving a grid and cathode, said cathode being cathode coupled to thecathode of said first amplifying tube; a second amplifying tube having acathode, a control grid and a plate, a second vacuum tube having a gridand a cathode, said cathode being cathode coupled to the cathode of saidsecond amplifying tube; testing terminal means for connecting theelements of a vacuum tube in a circuit, having a plate, a grid, andcathode terminal including a cathode terminal return; a direct-currentindicator connected for differential action between the plates of saidfirst and second amplifying tubes; radio-frequency bypass means foreliminating the radio-frequency present at the plates of the amplifyingtubes; non-reactive impedance means series connected in the cathodeterminal return of said testing terminal means; a source of threealternating voltages all of the same frequency, two having the sameamplitude, said three voltages having a phase relationship such that agiven phase angle between said two equal amplitude voltages is bisectedby the phase of the third voltage, said two equal amplitude voltagesbeing fed to the grids of said first and second vacuum tubesrespectively and said third voltage being fed to the grid terminal ofsaid testing terminal means; and a direct-current voltage source for theenergization of all of said tubes and said testing terminal means.

2. The apparatus as set forth in claim 1 in which the source of thethree alternating voltages provides voltages all of the same frequencyand the same amplitude, said three voltages having a phase relationshipsuch that a given phase angle between two of said voltages is bisectedby the phase of the third voltage, said two voltages being fed to thegrids of said first and second vacuum tubes respectively, and said thirdvoltage being fed to the grid terminal of said testing terminal means.

3. The apparatus as set forth in claim 1 in which the source of thethree alternating voltages provides voltages all of the same frequency,two

having the same amplitude, said three voltages being phase-related, saidtwo voltages being fed to the grids of said first and secondvacuum tubesrespectively, and said third voltage being fed to the grid terminal ofsaid testing terminal "means,

4 The apparatus as set forth in claim 1 in which thesource of thethreealternating voltages provides voltages all of the frequency and the sameamplitudajsaid three voltages being 120 phase-related, and being fed tothe grids of said vacuum tubes andgrid terminal of said testing terminalmeansrespeotively.

5. The apparatusas set forth in claim 1 and sensitivity adjusting meansfor the direct-current indicator. l

6. Inaftesting circuit: a pair of vacuum tubes each including a cathode,a grid; a plate load circuit, a'r 1d a direct-current indicatorconnected for differential action betwee n s'aid plate circuits; a pairof amplifying tubeseach having a plate, a cathode, and a grid;resistance means coupling the cathodes of said amplifying tubes to thecathodes of said vacuum tubes, respectively; a plurality of testterminals adapted to operatively connect a vacuum tube to be tested, oneterminal for each of the plate, grid, and cathode of the tube to betested; means connecting the cathode test terminal to the grids of saidvacuum tubes; a non-reactive impedance series connected between thecathode and plate test terminals; a source of three alternating voltagesof the same frequency, at least two having the same amplitude, the thirdvoltage having a phase angle which bisects the phase angle formedbetween said two voltages; means for feeding said two voltages to thegrids of said amplifying tubes, respectively; and means for feeding saidthird voltage to the grid test terminal.

7. In a testing circuit, the combination comprising: a pair of vacuumtubes each having a cathode, a grid, and a plate load circuit; adirectcurrent indicator connected for differential action between saidplate circuits; a plurality of test terminals adapted to operativelyconnect a vacuum tube to be tested therebetween, one terminal for eachof the plate, grid and cathode of the tube to be tested; meansinterconnecting the cathode test terminal with the grids of said pair oftubes; a non-reactive impedance series connected between the cathode andplate test terminals; 2. source of three alternating voltages of thesame frequency and at least two having the same amplitude, the phaseangle of the third voltage bisecting the angle formed between said twovoltages; means for applying said third voltage to the grid testterminal; and means for applying said two voltages to the cathodes ofsaid pair of tubes, respectively.

8. In a testing circuit, the combination comprising: a pair of vacuumtubes each having a cathode, a grid and a plate load circuit; adirectcurrent indicator connected between said plate circuits; aplurality of test terminals one for each of the plate, grid, and cathodeof a tube to be tested, the cathode terminal being connected to thegrids of said pair of tubes; a non-reactive impedance serially connectedbetween the oathode and plate terminals; a source of three alternatingvoltages of the same frequency and at least two having the sameamplitude, the phase angle of the third voltage bisecting the angleformed between said two voltages; means for applying said third voltageto the grid test terminal;

asiopea' and means for applying said two voltages to the cathodes ofsaid pair of tubes, respectively.

9. The combination according to claim 8, wherein said three alternatingvoltages are 120 phase related.

10. In a testing circuit, the combination comprising: a pair of vacuumtubes each having an input circuit and an output circuit; adirectcurrent indicator connected between the output circuits; aplurality of test terminals adapted to be connected to the input andoutput circuits of a tube to be tested; a non-reactive impedance seriesconnected between the output test terminals; means for applying thepotential across said impedance to the input circuits of each of saidpair of tubes; a source of three alternating voltages of the samefrequency and at least two having the same amplitude, the phase angle ofthe third voltage bisecting the angle formed between said two voltages;means for applying said third voltage to the input circuit testterminals and means for applying said two voltages to the input circuitsof said pair of tubes, respectively.

11. In a testing circuit, a pair of vacuum tubes each having a cathode,a grid and a plate load circuit, a direct-current indicator connectedbetween said plate circuits, a source of three alternating voltages ofthe same frequency and at least two having the same amplitude, eachvoltage being phase displaced 120 from the others, means for applyingsaid two voltages to the cathodes of said pair of tubes, respectively,and means for applying said third voltage to each grid of said pair oftubes, said last-named means including a non-reactive impedance and aplurality of terminals connected in series therewith, said terminalsbeing adapted to be connected to the elements of a tube to be tested.

CHARLES E. DENNIS.

No references cited.

